1. Field of the Invention
The present invention relates to design of a window permitting a high-power laser beam and a high-energy particle beam to enter or exit a region filled with gas from or to a vacuum region with the two beams in close proximity to one another while not permitting gas leakage into the vacuum region. The invention further relates to a window for a gas cell having high optical quality such that it permits transmission of a laser beam with minimal losses as well as particle beam transmission with minimum scattering and loss of beam current and is yet highly resistant to degradation by either the laser beam or particle beam.
2. Description of Prior Art
Energy or momentum exchange between free electrons and photons, such as that achieved in the free electron laser (FEL) requires, among others, phase-matching of the particles with the electromagnetic field in which energy and momentum are conserved. One method to achieve momentum exchange is the inverse Cerenkov effect, wherein a gaseous medium retards the phase velocity of the electromagnetic wave. This is depicted as prior art in FIG. 1, in which the phase velocity of a laser light wave 10 along one direction 12 in the medium is v.sub.1 =c/n, where c is the velocity of light in vacuum and n is the index of refraction of the medium. An electron 14 traveling with velocity v.sub.e in another direction 16 intersects the laser beam 10 at an angle .theta.. To insure that the electron 14 remains in a field of constant phase, the phase velocity of the laser in the direction of the electron's velocity, v.sub.1 /cos .theta.=c/n cos.theta., must equal the velocity of the electron 14, .beta.c, where .beta.=v.sub.e /c. This is satisfied when EQU .theta.=cos.sup.-1 (1/.eta..beta.)
and is referred to as the Cerenkov angle, .theta..sub.c. The electron and interacting photon must further obey the energy conservation relation, EQU E.sub.f -E.sub.i =h .omega.
where E.sub.i and E.sub.f are the initial and final electron energies respectively, and h .omega. is the photon energy (.omega. is the laser frequency); and the momentum conservation relation EQU .rho..sub.f -.rho..sub.i = k
where .rho..sub.i and .rho..sub.f are the initial and final electron momenta respectively, and k is the laser wavevector, given by .vertline.k.vertline.=.omega.n/c, intersecting .rho..sub.i at an angle .theta.. Thus for photon energies much less than the electron energy we find EQU E.sub.f -E.sub.i .perspectiveto..beta.c(.rho..sub.f -.rho..sub.i).perspectiveto. .omega.n.beta. cos .theta.
illustrating satisfaction of both energy and momentum conservation conditions.
The net effect of this inverse Cerenkov interaction between the electrons 14 and the electric field of the laser 10 is momentum, energy, and velocity modulation of the electron beam. The energy exchange can be much greater than the random energy spread of the electron beam. By allowing the electrons to drift, the velocity modulation of the electrons 14 is transformed into a density modulation, creating electron bunches separated by the laser wavelength. Coherent optical radiation can be extracted from the bunched beam, thus creating in a manner analogous to microwave devices an optical klystron or optical traveling wave tube. This coherent radiation from the bunched beam will contain light at the higher harmonics of the laser frequency, hence harmonic generation is feasible, possibly extending up to the ultraviolet region. Another application is in the area of laser-driven particle accelerators in which multiple passes of the electron through the laser field can be used to accelerate a portion of the electrons to high energies. The very high electric fields obtainable from lasers means large acceleration gradients (.perspectiveto.10.sup.2 -10.sup.3 MeV/m) are possible.
The advantage of using electron accelerator systems as sources of coherent optical radiation is due to the high powers and energies per pulse such systems are capable of providing at relatively high efficiencies. For the optical klystron, just as in the microwave klystron amplifier, the lack of a direct feedback path between the input and output signals enables high power gains, whereas in quantum amplifiers this is limited because of the onset of spurious oscillations. An additional advantage of electron beam devices is that free electrons can emit over and be coupled to a wide and continuous electromagnetic spectrum.
However, most approaches utilizing energy exchange between photons and relativistic electrons are conducted entirely in a vacuum. This frequently results in highly complex and expensive equipment. The advantage of intermixing of the two beams as described above using the inverse Cerenkov interaction has been recognized theoretically for some time but lack of appropriate equipment has prevented exploration of the technique. The instant invention permits fabrication of a gas cell having windows for ingress/egress of the two beams in a manner permitting effective use of the technique.